1. Field of the Invention
The present invention relates to a light-source device and optical scanning device having a light-source angle adjusting function and for use in an image forming apparatus, such as a digital copier, facsimile, or laser printer, and also relates to an image forming apparatus having incorporated therein the light-source device or optical scanning device.
2. Description of the Related Art
An image forming apparatus, such as a digital copier, facsimile, and laser printer, is provided with various optical scanning devices for scanning a photosensitive member with a light beam. In the optical scanning devices that have been conventionally used, a polygon mirror or galvanometer mirror has been used as a deflector that deflects a light beam from a light source. However, to form an image with higher resolution within a short time, such a polygon mirror or galvanometer mirror has to be rotated at higher speed. Rotation of the polygon mirror or galvanometer mirror with high speed has a limitation due to durability of a bearing rotatably supporting the polygon mirror or galvanometer mirror explained above, heating at the time of rotation, noise, and other factors.
To get around this problem, for use as deflectors in the optical scanning device, deflectors using silicon micromachining have been suggested in recent years in, for example, Japanese Patent No. 2924200, Japanese Patent No. 3011144, Japanese Patent Application Laid-Open Publication No. 2002-82303, Japanese Patent No. 3445691, and Japanese Patent No. 3543473. In a deflector 501 of this type, as depicted in FIG. 54, a vibrating mirror 502 with its surface serving as a deflector plane 502a and a torsional bar 503 pivotally supporting the vibrating mirror 502 are integrally formed in the deflector 501. With the deflector 501, the vibrating mirror 502 can be downsized, thereby downsizing the deflector itself. In addition, since the vibrating mirror 502 is vibrated in a reciprocating manner by using the resonance of the vibrating mirror 502, a high-speed operation can be advantageously performed with low noise and power consumption.
Furthermore, with low vibration and little heating, the housing that accommodates the optical scanning device and others can be made thinner. Therefore, even if the housing is configured of a low-cost resin molding material with a small ratio of mixture of glass fiber, it is an advantage that an influence on image quality hardly occurs. In particular, Japanese Patent Application Laid-Open Publication No. 2002-82303 discloses an example in which the deflector 501 explained above is used in place of a polygon mirror. Also, Japanese Patent No. 3445691 and Japanese Patent No. 3543473 disclose image forming apparatuses in which a vibrating mirror is used in place of a polygon mirror to achieve low noise and power consumption, which is suitable for office environment and also earth environment.
However, when the vibrating mirror 502 explained above is driven, a deformation in active plane occurs as explained below, due to the moment of inertia and resilience of the vibrating mirror 502.
When the dimension of the vibrating mirror 502 depicted in FIG. 54 is such that its length is 2a, its width is 2b, and its thickness is d, the length of the torsional bar 503 is L and its width is c, the density of Si is ρ, and the material constant is G, a moment of inertia I of the vibrating mirror 502 is represented by Equation (1).I=(4abρd/3)×a2  (1)
As represented in Equation (1) above, the local moment of inertial I of the vibrating mirror 502 is a function of a distance from a rotational axis of the vibrating mirror 502, and it can be found that, as the distance from the rotational axis is increased, the moment of inertia is increased. Furthermore, since the thickness of the vibrating mirror 502 itself is as thin as several hundred micrometers, with a change in rotation speed associated with the reciprocating movement and an inertial force on the vibrating mirror 502, forces in opposite directions are exerted at a position near the torsional bar 503 of the vibrating mirror 502 and an end away from the torsional bar 503, thereby causing, as depicted in FIG. 55, the vibrating mirror 502 to be deformed to become wavy. Therefore, wave aberration of a bundle of light beams reflected by the vibrating mirror 502 is increased, thereby making the optical beam thick.
FIG. 55 depicts a deformed state of the vibrating mirror 502 with a simple plate shape. In FIG. 55, a deterioration in wave aberration of light beams and also a shift in an incident position in a direction orthogonal to the torsional bar 503 (a main scanning direction) as represented by a broken line occur at the same time. In this case, since an apparent curvature differs, a shift (shift in focus) occurs in an image-forming position of the light beams. In particular, as depicted in FIGS. 56 and 57, when the light beams converge on an edge of the vibrating mirror 502 due to assembling error of the deflector, the light source, and other components, the light beams may become thick (see FIG. 57), or a shift in focus may occurs (see FIG. 56).
Also, the light beams converging on the edge of the vibrating mirror 502 become a light-gathered bundle in the main scanning direction (see FIG. 56) or a diffused light bundle (see FIG. 57), and therefore the light beams cannot be uniformly gathered at the image-forming position. For this reason, a desired beam spot size cannot be achieved. Therefore, in conventional examples, light beams cannot be gathered over the entire scanned plane, thereby making it impossible to uniformly keep the beam spot size and, as a result, disadvantageously leading to image deterioration.
Moreover, as for the resonant frequency, there is a problem in which a change in spring constant of the torsional bar due to temperature or a change in viscosity resistance of air due to atmospheric pressure may change a deflection angle.
To get around this problem, as disclosed in Japanese Patent Application Laid-Open Publication No. 2004-279947, one suggested control is such that the deflection angle is detected by detecting a beam for use in scanning, thereby adjusting a current applied to the vibrating mirror 502 and stably keeping the deflection angle.
However, as a method of reducing deformation of the vibrating mirror 502, if the flexural rigidity of the board of the vibrating mirror 502 is increased, that is, if the thickness of the board of the vibrating mirror 502 is increased, the mass of the vibrating mirror 502 is also increased. Therefore, with comparison in the deflection angle of the vibrating mirror 502 with the same scanning frequency, the deflection angle of the vibrating mirror 502 with an increased thickness is disadvantageously decreased. For this reason, simply increasing the thickness cannot solve the problem.